This invention pertains to CMP polishing compositions comprising silica abrasive, as well as methods of polishing substrates with such polishing compositions.
Compositions and methods for planarizing or polishing the surface of a substrate are well known in the art. Polishing compositions (also known as polishing slurries) typically contain an abrasive material in an aqueous solution and are applied to a surface by contacting the surface with a polishing pad saturated with the polishing composition. Typical abrasive materials include silicon dioxide, cerium oxide, aluminum oxide, zirconium oxide, and tin oxide. U.S. Pat. No. 5,527,423, for example, describes a method for chemically-mechanically polishing a metal layer by contacting the surface with a polishing slurry comprising high purity fine metal oxide particles in an aqueous medium. The polishing slurry is typically used in conjunction with a polishing pad (e.g., polishing cloth or disk). Suitable polishing pads are described in U.S. Pat. Nos. 6,062,968, 6,117,000, and 6,126,532 which disclose the use of sintered polyurethane polishing pads having an open-celled porous network and U.S. Pat. No. 5,489,233 which discloses the use of solid polishing pads having a surface texture or pattern. Alternatively, the abrasive material may be incorporated into the polishing pad. U.S. Pat. No. 5,958,794 discloses a fixed abrasive polishing pad.
Conventional polishing systems and polishing methods typically are not entirely satisfactory at planarizing semiconductor wafers. In particular, polishing compositions and polishing pads can have less than desirable polishing rates or polishing selectivities, and their use in chemically-mechanically polishing semiconductor surfaces can result in poor surface quality. Because the performance of a semiconductor wafer is directly associated with the planarity of its surface, it is crucial to use a polishing composition and method that results in a high polishing efficiency, selectivity, uniformity, and removal rate and leaves a high quality polish with minimal surface defects.
The difficulty in creating an effective polishing system for semiconductor wafers stems from the complexity of the semiconductor wafer. Semiconductor wafers are typically composed of a substrate, on which a plurality of transistors has been formed. Integrated circuits are chemically and physically connected into a substrate by patterning regions in the substrate and layers on the substrate. To produce an operable semiconductor wafer and to maximize the yield, performance, and reliability of the wafer, it is desirable to polish select surfaces of the wafer without adversely affecting underlying structures or topography. In fact, various problems in semiconductor fabrication can occur if the process steps are not performed on wafer surfaces that are adequately planarized.
The use of alcohols in polishing compositions is well known in the art. For example, U.S. Pat. Nos. 5,391,258 and 5,476,606 disclose a polishing composition comprising an abrasive and an anion containing two acid groups (e.g., hydroxyl groups), which purportedly controls the rate of removal of silica. U.S. Pat. No. 5,614,444 discloses polishing compositions comprising polishing additives having a polar component (e.g., an alcohol) and a non-polar component (e.g., an alkyl group). The polishing additive is used to suppress the removal rate of a dielectric material. U.S. Pat. No. 5,733,819 discloses a polishing composition comprising silicon nitride abrasive, water, acid, and optionally a water-soluble alcohol additive (e.g., ethanol, propanol, ethylene glycol). U.S. Pat. No. 5,738,800 discloses an aqueous polishing composition comprising abrasive, a surfactant, and a complexing agent comprising two functional groups (e.g., hydroxyl groups), which purportedly complexes silica and silicon nitride layers. U.S. Pat. No. 5,770,103 discloses a polishing composition comprising mono-, di-, or tri-substituted phenol compounds, which purportedly increase the removal rates of titanium substrate layers. U.S. Pat. No. 5,895,509 discloses a polishing composition comprising an abrasive, isopropyl alcohol, and water. U.S. patent application Ser. No. 2001/0013506 discloses a polishing composition comprising abrasive particles, an oxidizer, a pH of about 5 to about 11, and optionally an organic diluent (e.g., methanol, ethanol, ethylene glycol, or glycerol). EP 1 150 341 A1 discloses the use of alcohols as dissolution promoters in polishing compositions comprising a film-forming agent. JP 11116942 discloses a polishing composition comprising silica, water, a water-soluble polymeric compound, a base, and a compound having 1-10 alcoholic hydroxyl groups. JP 2000230169 discloses an aqueous polishing composition comprising silica, a pH buffer, and a water-soluble polishing accelerator (e.g., an alcohol), which purportedly improves polishing rates. WO 98/48453 discloses a polishing composition comprising spherical silica, an amine hydroxide, and an alkaline liquid carrier comprising up to about 9% alcohol, which purportedly increases polishing rates. WO 01/84613 discloses the use of a fixed abrasive article and an aqueous polishing composition comprising a polar component (e.g., methanol, ethanol, etc.), which purportedly reduces the surface tension of the polishing composition and provides better wetting of the surface of hydrophobic substrates.
There remains a need, however, for alternative polishing compositions, which exhibit satisfactory polishing characteristics, such as good removal rates and low surface defects. The invention seeks to provide such a polishing composition and method. These and other advantages of the invention will be apparent from the description of the invention provided herein.
The invention provides a polishing composition comprising (a) a silica abrasive, (b) methanol, and (c) a liquid carrier, wherein the polishing composition has a pH of about 1 to about 6 and the interaction between the silica abrasive and the methanol provides colloidal stability to the polishing composition. The invention also provides a method for polishing a substrate comprising a silicon-based dielectric layer using the polishing composition. The invention further provides a method of stabilizing silica abrasive by contacting the abrasive with methanol.
The invention is directed to a polishing composition comprising silica abrasive, methanol, and a liquid carrier. The polishing composition has a pH of about 1 to about 6. The interaction between the silica abrasive and the methanol provides colloidal stability to the polishing composition.
The silica abrasive typically is selected from the group consisting of fumed silica, colloidal silica, silica-coated abrasive particles, silica-containing co-formed particles (e.g., aluminosilicates), and combinations thereof. Silica-coated abrasive particles can include silica-coated alumina or silica-coated polymer particles. The silica-containing co-formed particles typically contain about 10 wt. % or more silicon. Any suitable amount of silica can be present in the polishing composition. The polishing composition typically comprises silica abrasive in an amount of about 0.1 wt. % or more (e.g., about 0.5 wt. % or more). The polishing composition typically also comprises silica abrasive in an amount of about 5 wt. % or less (e.g., about 2 wt. % or less or about 1 wt. % or less).
The interaction of the silica abrasive with the methanol under acidic conditions results in a polishing composition that is colloidally stable. Colloidal stability refers to the maintenance of the suspension of abrasive particles over time. In the context of this invention, the polishing composition is considered colloidally stable if, when 100 ml of the polishing composition placed in a 100 ml graduated cylinder and allowed to stand unagitated for a time of 2 hours, the difference between the concentration of particles in the bottom 50 ml of the graduated cylinder ([B] in terms of, for example, g/ml) and the concentration of particles in the top 50 ml of the graduated cylinder ([T] in terms of, for example, g/ml) divided by the initial concentration of particles in the abrasive composition ([C] in terms of, for example, g/ml) is less than or equal to 0.5 (i.e., {[B]xe2x88x92[T]}/[C]xe2x89xa60.5). The value of [B]xe2x88x92[T]/[C] desirably is less than or equal to 0.3, and preferably is less than or equal to 0.1.
The silica abrasive typically has an average particle size of about 500 nm or less, e.g., about 10 nm to about 500 nm. Preferably, the silica abrasive has an average particle size of about 250 nm or less, e.g., about 10 nm to about 250 nm. More preferably, the silica abrasive has an average particle size of about 150 nm or less, e.g., about 10 nm to about 150 nm or about 10 nm to about 120 nm.
Any suitable amount of methanol can be present in the polishing composition. The amount of methanol relative to the amount of silica abrasive should be sufficient to produce a polishing composition that is colloidally stable. Methanol typically is present in the polishing composition in an amount of about 100 ppm or more (e.g., about 200 ppm or more). Methanol typically also is present in the polishing composition in an amount of about 10,000 ppm or less (e.g., about 6000 ppm or less, about 4000 ppm or less, or about 2000 ppm or less). Preferably, the polishing composition comprises about 100 ppm to about 1000 ppm of methanol. More preferably, the polishing composition comprises about 100 ppm to about 800 ppm of methanol. The amount of methanol present in the polishing composition can be related to the amount of silica abrasive. For example, the polishing composition can comprise about 200 ppm to about 4000 ppm (e.g., about 200 ppm to about 2000 ppm, or about 200 ppm to about 1600 ppm) of methanol per wt. % of silica abrasive.
The polishing composition has an acidic pH of about 1 to about 6. Preferably, the polishing composition has a pH of about 1.5 to about 5 (e.g., about 1.5 to about 4). The polishing composition optionally further comprises pH adjusting agents, for example, potassium or ammonium hydroxide, mineral acids, or organic acids.
Silica abrasives are very stable at higher pH (e.g., above pH of 6). However, at acidic pH (e.g., below pH of 6), the silica particles have a lower surface charge suggesting that hydroxyl groups present on the silica particle are protonated. Under acidic conditions, the silica particles are more likely to collide, resulting in irreversible formation of siloxane bonds and aggregation. Treating the silica abrasive with a small amount of methanol results in stabilization (e.g., reduced aggregation) of the silica abrasives at low pH. Methanol reacts with the surface silanol groups under acidic conditions. Colloidal silica particles have more surface hydroxyl groups than fumed silica and thus are more difficult to stabilize at acidic pH.
A liquid carrier is used to facilitate the application of the silica abrasive, methanol, and any optional additives to the surface of a suitable substrate to be polished (e.g., planarized). The liquid carrier is typically an aqueous carrier and can be water alone, can comprise water and a suitable water-miscible solvent, or can be an emulsion. Preferably, the aqueous carrier consists of water, more preferably deionized water.
The polishing composition optionally further comprises an oxidizing agent. The oxidizing agent can be any suitable oxidizing agent. Suitable oxidizing agents include inorganic and organic per-compounds, bromates, nitrates, chlorates, chromates, iodates, iron and copper salts (e.g., nitrates, sulfates, EDTA, and citrates), rare earth and transition metal oxides (e.g., osmium tetraoxide), potassium ferricyanide, potassium dichromate, iodic acid, and the like. A per-compound (as defined by Hawley""s Condensed Chemical Dictionary) is a compound containing at least one peroxy group (xe2x80x94Oxe2x80x94Oxe2x80x94) or a compound containing an element in its highest oxidation state. Examples of compounds containing at least one peroxy group include but are not limited to hydrogen peroxide and its adducts such as urea hydrogen peroxide and percarbonates, organic peroxides such as benzoyl peroxide, peracetic acid, and di-tert-butyl peroxide, monopersulfates (SO52-), dipersulfates (S2O82-), and sodium peroxide. Examples of compounds containing an element in its highest oxidation state include but are not limited to periodic acid, periodate salts, perbromic acid, perbromate salts, perchloric acid, perchlorate salts, perboric acid, perborate salts, and permanganates. The oxidizing agent preferably is hydrogen peroxide.
The polishing composition optionally further comprises a chelating or complexing agent. The complexing agent is any suitable chemical additive that enhances the removal rate of the substrate layer being removed. Suitable chelating or complexing agents can include, for example, carbonyl compounds (e.g., acetylacetonates, and the like), simple carboxylates (e.g., acetates, aryl carboxylates, and the like), carboxylates containing one or more hydroxyl groups (e.g., glycolates, lactates, gluconates, gallic acid and salts thereof, and the like), di-, tri-, and poly-carboxylates (e.g., oxalates, phthalates, citrates, succinates, tartrates, malates, edetates (e.g., dipotassium EDTA), polyacrylates, mixtures thereof, and the like), carboxylates containing one or more sulfonic and/or phosphonic groups, and the like. Suitable chelating or complexing agents also can include, for example, di-, tri-, or polyalcohols (e.g., ethylene glycol, pyrocatechol, pyrogallol, tannic acid, and the like) and amine-containing compounds (e.g., ammonia, amino acids, amino alcohols, di-, tri-, and polyamines, and the like). Preferably, the complexing agent is a carboxylate salt, more preferably an oxalate salt. The choice of chelating or complexing agent will depend on the type of substrate layer being removed.
It will be appreciated that many of the aforementioned compounds can exist in the form of a salt (e.g., a metal salt, an ammonium salt, or the like), an acid, or as a partial salt. For example, citrates include citric acid, as well as mono-, di-, and tri-salts thereof; phthalates include phthalic acid, as well as mono-salts (e.g., potassium hydrogen phthalate) and di-salts thereof, perchlorates include the corresponding acid (i.e., perchloric acid), as well as salts thereof. Furthermore, certain compounds or reagents may perform more than one function. For example, some compounds can function both as a chelating agent and an oxidizing agent (e.g., certain ferric nitrates and the like).
The polishing composition can be prepared in any suitable manner. In general, the polishing composition described herein will be prepared, and a polishing composition comprising a silica abrasive and a liquid carrier will be stabilized by (i) providing silica abrasive, (ii) providing a liquid carrier for the silica abrasive, and (iii) contacting the silica abrasive with methanol to form a stabilized polishing composition, desirably at a pH of about 1 to about 6. The methanol can contact (and interact with) the silica abrasive at any suitable point in the preparation (or stabilization) of the polishing composition. Thus, for example, the methanol can be added to a polishing composition comprising silica abrasive and a liquid carrier. Alternatively or in addition, the methanol can contact the silica abrasive before the silica abrasive is added to a liquid carrier.
The polishing composition described herein can be used to polish (e.g., planarize) a substrate. The method of polishing a substrate comprises (i) providing the polishing composition, (ii) contacting the substrate with the polishing composition, and (iii) abrading at least a portion of the substrate with the polishing composition to polish the substrate. The polishing composition desirably is used in a method of polishing a substrate comprising at least one dielectric layer, whereby the substrate is contacted with the polishing composition and at least a portion of the dielectric layer of the substrate is abraded such that the dielectric layer becomes polished. The substrate can be any suitable substrate (e.g., an integrated circuit, memory or rigid disks, metals, ILD layers, semiconductors, micro-electro-mechanical systems, ferroelectrics, magnetic heads, polymeric films, and low and high dielectric constant films) and can contain any suitable dielectric layer (e.g., insulating layer). Typically the dielectric layer is a silicon-based material, for example, silicon dioxide. The dielectric layer also can be a porous metal oxide, glass, organic polymer, fluorinated organic polymer, or any other suitable high or low-xcexa dielectric layer. The dielectric layer preferably comprises silicon oxide, silicon nitride, silicon oxynitride, silicon carbide, aluminum oxide, or a material with a dielectric constant of about 3.5 or less. The substrate typically further comprises one or more layers comprising a metal or a polishing stop layer (e.g., silicon nitride). For example, the metal can be tungsten, copper, aluminum, or nickel. Preferably, the metal is tungsten or copper. The polishing composition can provide good polishing efficiency, selectivity, uniformity and/or removal rates as well as reduced erosion of the substrate surface and fewer surface defects as compared to many conventional polishing compositions.
The following examples further illustrate the invention but, of course, should not be construed as in any way limiting its scope.